Emulsion polymerization and suspension polymerization are known as production methods of fluoropolymers. In these polymerization methods, perfluorocarbon (PFC) or chlorofluorocarbon (CFC) has been used as a polymerization medium. PFC and CFC however have a high global warming potential (GWP), and CFC also has a high ozone depletion potential. Therefore, it is preferred to reduce the amounts of these to be used. Further, since PFC and CFC are likely to adsorb on fluoropolymers, they are released and vaporized from fluoropolymers to form bubbles in the fluoropolymers when the fluoropolymers are extruded. As a result, PFC and CFC causes molding failures. Therefore, solvents alternative to PFC and CFC have been suggested.
Patent Literature 1 suggests a polymerization method that includes bringing a fluoromonomer into contact with a solvent that contains carbon; fluorine; at least one hydrogen atom; at least as many fluorine atoms as hydrogen atoms; no more than two adjacent —CH2— groups; no hydrogen atoms on any primary carbon; and no methyl (—CH3) groups, except as the grouping —CF2OCH3.
Patent Literature 2 suggests, in order to produce a fluoropolymer by polymerization in a polymerization medium, a production method of a fluoropolymer, in which (perfluoroalkyl)alkyl ether such as F(CF2)4OCH3, F(CF2)4OC2H5, (CF3)2CFOCH3, and F(CF2)3OCH3, is used as a polymerization medium.
Further, in the above described production methods of fluoropolymers using polymerization mediums, it is preferred that the polymerization medium is separated, recovered, and recycled after a polymerization reaction in view of economic and environmental benefits. Thus, a polymerization medium recovery method is known.
Patent Literature 3 discloses a production method of a fluorocopolymer including copolymerizing tetrafluoroethylene with a specific fluorine-containing vinyl ether using perfluoroheptane as a reaction solvent in the presence of a chain transfer agent in an amount of 0.5 to 4 mol % based on the total amount of the fed monomers, and discloses recovery of the reaction solvent after the polymerization reaction in the method.
Patent Literature 4 discloses a method for producing fluoropolymers, comprising polymerizing at least one fluorinated monomer to yield a fluoropolymer in a polymerization medium including water and a specified low-telogenic hydrofluoroether (HFE), and the method further comprises separating the fluoropolymer and water from the HFE solvent by distillation of the HFE solvent while the fluoropolymer is agglomerated, and reusing the HFE solvent for another polymerization method.